1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to protective wiring devices.
2. Technical Background
Electrical distribution systems that provide power to structures such as residences, commercial buildings or other such facilities typically include one or more breaker panels coupled to a source of AC power. Of course, the breaker panel distributes AC power to one or more branch electric circuits installed in the structure. The electric circuits may typically include one or more receptacle outlets and may further transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. The receptacle outlets provide power to user-accessible loads that include a power cord and plug, the plug being insertable into the receptacle outlet. However, certain types of faults have been known to occur in various portions of the electrical distribution systems. Accordingly, electric circuit protection devices may be disposed throughout the distribution system, i.e., in the breaker panel and in protective devices having receptacle outlets. Protective devices may also be installed in the electrical load itself.
Both receptacle wiring devices and electric circuit protective wiring devices may be disposed in an electrically non-conductive housing. The housing includes electrical terminals that are electrically insulated from each other. Line terminals couple the wiring device to conductors that provides electrical power from the electrical distribution system. Load terminals are coupled to wiring that directs AC power to one or more electrical loads. Those of ordinary skill in the pertinent art will understand that the term “load” refers to an appliance, a switch, or some other electrically powered device. Load terminals may also be referred to as “feed-through” terminals because the wires connected to these terminals may be coupled to a daisy-chained configuration of receptacles or switches. The load may ultimately be connected at the far end of this arrangement. The load terminals may also be connected to an electrically conductive path that is also connected to a set of receptacle contacts. The receptacle contacts are in communication with receptacle openings disposed on the face of the housing. This arrangement allows a user to insert an appliance plug into the receptacle opening to thereby energize the device.
As noted above, there are several types of electric circuit protection devices. For example, such devices include ground fault circuit interrupters (GFCIs), ground-fault equipment protectors (GFEPs), and arc fault circuit interrupters (AFCIs). This list includes representative examples and is not meant to be exhaustive. Some devices include both GFCIs and AFCIs. As their names suggest, arc fault circuit interrupters (AFCIs), ground-fault equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs) perform different functions. An arc fault typically manifests itself as a high frequency current signal. Accordingly, an AFCI may be configured to detect various high frequency signals and de-energize the electrical circuit in response thereto. A ground fault occurs when a current carrying (hot) conductor creates an unintended current path to ground. A differential current is created between the hot/neutral conductors because some of the current flowing in the circuit is diverted into the unintended current path. The unintended current path represents an electrical shock hazard. Ground faults, as well as arc faults, may also result in fire. A “grounded neutral” is another type of ground fault. This type of fault may occur when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded.
When a device is installed, its line terminals are connected to an AC power source, such as a single phase 120 VAC AC power source. However, transient voltages may propagate in an electrical distribution system as well as the AC power signal. Further, the amplitudes of transient voltages are typically greater than the amplitude of the source voltage by at least an order of magnitude. Transient voltage pulses may be generated by any number of events. For example, transient voltages may be introduced into the distribution system by lightning. Transient voltages may also be generated when an inductive load is turned off, when a motor with noisy brushes is operated, or by other such load situations.
Transient voltages are known to damage protective devices such that the device will cease to function as designed. This is sometimes referred to as an end of life condition. End of life failure modes include failure of device circuitry, the relay solenoid that opens the GFCI interrupting contacts, and/or the solenoid driving device, such as a silicon controlled rectifier. The damage may result in the protective device permanently denying power to the protected portion of the electric circuit. In this case, the user is forced to replace the protective device. Alternatively, the damage may result in the protective device still providing power to the load even though the device has become non-protective. In this case, the user is left unprotected after an end-of-life condition has occurred. Thus the user is either inconvenienced by having to change out the device, or even worse, he is left unprotected.
Most devices include surge protection components. However, surge protection components occupy a considerable volume within the device housing. As a result, the overall size of the device is relatively large, making it harder to install the device within a wall box. Another problem is that surge protective components themselves are known to experience an end-of-life condition. If the surge protection component fails, the device is unprotected from damages due to transient voltages.
Accordingly, a compact protective device that includes an improved space-conserving surge protection arrangement is needed that continues to afford protection after the occurrence of a voltage transient event on the electrical distribution system. The compact protective device must be configured to reliably protect the user from a fault condition in the electrical power distribution system. Further, a protective device is needed that is equipped to decouple the load terminals from the line terminals in the event of an end of life condition.